The moment a player has just one card they must yell “UNO!”. If they are caught not saying “Uno” by another player before any card has been played, the player must draw two new cards.
The objective of the game UNO is to be the first player to score 500 points. Points are scored by ridding yourself of all the cards in your hand before your opponent(s). You score points for cards left in your opponent's hands.
To start the game, each player is dealt 7 cards with the remaining ones placed face down to form a DRAW pile. The top card of the DRAW pile is turned over to begin a DISCARD pile.
The first player has to match the card in the DISCARD pile either by number, color or word. For example, if the card is a red 7, player must throw down a red card or any color 7. Or the player can throw down a Wild Card. If the player doesn't have anything to match, he must pick a card from the DRAW pile. If he can play what is drawn, great. Otherwise play moves to the next person.
When you have one card left, you must yell "UNO" (meaning one). Failure to do this results in you having to pick two cards from the DRAW pile. That is, of course if you get caught by the other players. Once a player has no cards left, the hand is over. Points are scored (see scoring section) and you start over again. That's UNO in a nutshell.
Learn more on how to play UNO at Wonkavator.
Tip! Learn Best Strategies to win Uno from Uno Rules.